By Moris Amon
Isotactic homo-polypropylene (PP) is a semicrystalline polymer of great commercial importance. In its solid state, microscopic crystalline and amorphous domains intermingle. The crystalline unit cell is usually monoclinic. This crystal configuration is called the α (alpha) form. There is also a β (beta) form with hexagonal unit cells, which is usually present in small quantities, if at all – unless special processing conditions or special nucleating agents described in this article are used. The physical properties of the α and β forms differ, and this difference can be used to advantage in certain applications, as will be explained.
How can β crystals be detected?
The most accurate way of determining the β content of a PP sample is wide-angle x-ray diffraction (WAXD). The β phase shows a single strong diffraction peak due to the (300) crystal plane, while the α phase shows three strong peaks due to the (110), (130), and (040) planes. The fraction of β in the overall crystalline phase, called the “K value” can be calculated as the ratio of the intensity of the (300) peak to the sum of the intensities of all four peaks. When K=0, all the crystals are α; when K=1, they are all β.
Not all analytical labs have WAXD instruments. A more commonly available technique is differential scanning calorimetry (DSC) whereby the temperature of a small sample is raised at a controlled, constant rate (usually expressed in °C / min), and the variable heat input (W/g) needed to maintain this rate is measured. The heat input will show peaks (endotherms) when crystals melt. These peaks are broad because the melting points of individual crystals depend on their size and their level of perfection, both of which exhibit a range in industrial PP.
However, the locations of the peak maxima along the temperature axis are distinct for the α and β phases: typical values are 165-168°C for the α and 152-155°C for the β. The ratio of the area under the β peak to the total area under both peaks gives an alternative K value. Because the broad DSC peaks overlap, whereas the narrow WAXD peaks do not, the latter method is more accurate. Examples of DSC scans of purely α and mixed α+β PP are shown in Figures 1 and 2, respectively.
How can one generate more β crystals?
In most common PP products, whether extruded, blown, spun or molded, the K value is below 0.1 and often close to zero. Researchers have found that it can be raised by crystallizing the polymer from the melt very slowly at high temperatures (above 120°C). This is not practical since processors usually want to run faster, not slower, for economic reasons. There is also evidence indicating that crystallization at high shear rate favors β crystals slightly, (e.g. where the polymer flows rapidly very close to a mold wall), but such shear rates cannot be obtained throughout the thickness of a product.
The practical way to raise the K value is to use special nucleating agents. Nucleating agents are additives finely dispersed into the polymer at low concentrations to seed crystals during solidification from the melt. They increase the speed of crystallization, and often, the final crystallinity (i.e. the ratio of crystalline mass to the total of crystalline and amorphous masses.) The well-known nucleating agents, such as sodium benzoate and sorbitol derivatives, help the formation of α crystals only. The specific β nucleating agents reported in the literature include the following:
- Quinacridone dye known as “red E3B” (imparts a light pink coloration to the product.)
- Aluminum salt of 6-quinazirin sulfonic acid.
- Disodium salt o-phthalic acid.
- Isophthalic and terephthalic acids.
- N-N’-dicyclohexyl 2-6-naphthalene dicarboximide, known as “NJ Star NU-100”.
- A blend of organic dibasic acid + oxide, hydroxide, or acid of Group II metal (Mg, Ca, St, Ba.)
In addition, there are masterbatches of proprietary β nucleating agents sold by the Mayzo Corp. of Norcross, GA, which are claimed to be more effective than the above in company publications. K values as high as 0.9, produced without extreme processing conditions, have been reported in the literature.
What are the properties of β crystalline PP?
When comparing α crystalline PP to predominantly β crystalline PP, the latter is found to show the following differences:
- Lower Young’s modulus.
- Lower yield stress.
- Higher elongation at yield.
- Higher elongation at break.
- Higher impact strength.
While these properties are not necessarily desirable in a finished product, they can greatly improve the draw (stretch) ability of PP sheet at temperatures where it would normally draw very unevenly or break.
Other key findings pertaining to PP with a finite β crystalline content:
- When subjected to tensile stress at temperatures below the melting point of the β phase, β crystallites convert to α. The conversion is accompanied by cavitation – i.e. the formation of numerous microscopic voids.
- When maintained at temperatures above 120°C, β crystallites convert to α. The conversion accelerates as temperature increases.
What are the application benefits?
Thermoforming is used widely to manufacture inexpensive containers such as cups and tubs. PP is an ideal material for these applications because of its low cost and desirable physical properties. Unfortunately, α crystalline PP sheet is difficult to thermoform because of the abrupt drop in stiffness near but below its melting point, right in the middle of its forming temperature range. If the temperature is even slightly too high, the sheet tends to sag as it is preheated, before it can be clamped in the mold. If the temperature is lowered to avoid this problem, then the sheet does not draw uniformly, resulting in excessive thickness variation in the finished article, up to outright holes. Therefore, the process window, if it can be found at all, is very narrow.
To widen the process window to a comfortable level, PP sheet with a K value between 0.5 and 0.8 is thermoformed just above the β melting point but safely below the α melting point. Then, the β phase is converted to a during the draw. By the time the process is finished, the product is predominantly α crystalline. In this way,
- sheet sag is avoided because of the lower forming temperature and unmelted α crystalline phase,
- the final thickness distribution is uniform because of the better draw ability of the β phase,
- and the lower stiffness of β crystalline PP is avoided in the final product.
The only drawback is that if a transparent final product is desired, the haze will be higher than that of α PP stretched to the same extent. This is not an issue in opaque products.
Cavitated / Microporous Films
PP films with numerous closed-cell microvoids are favored in many packaging applications for their opacity, attractive pearlescent appearance, elimination or reduction of costly white pigment (TiO2) usage, or high gas permeability. They are normally manufactured by incorporating incompatible cavitating agent particles in the polymer and stretching extruded sheet uniaxially or biaxially. Fractures initiated at the cavitating agent-polymer interfaces grow into microvoids as the sheet is stretched. Known cavitating agents include calcium carbonate (CaCO3) and PP-incompatible, higher-melting polymers such as polybutylene terephthalate (PBT) or Nylon. These agents add to the cost of the film and are difficult to disperse as uniformly and finely as necessary to reach opacity targets throughout the product. Moreover, the most common agent, CaCO3, is almost 3 times denser than PP and reduces the film’s yield (m2/kg.)
The alternative is to make film with high β crystallinity (K > 0.8) and stretch it below the melting point of the β phase. Microvoids will form spontaneously as the β crystallites are converted to α. By the time the process is completed, most of the β phase will disappear, thus avoiding its lower stiffness.
Experiments have shown that especially when a β nucleating agent is added to PP impact copolymer instead of homopolymer, film of high opacity and low bulk density can be obtained in the absence of any cavitating agent. PP impact copolymer is a block copolymer of propylene and ethylene, where some cavitation occurs upon stretching in any event because of micro-fractures initiated at the block boundaries. This effect and β crystallinity in the PP blocks are synergistic.
Note that the β-induced cavitation mechanism can also be used in thermoforming by lowering the sheet temperature below the melting point of the β phase. This approach will produce white opaque containers.
Impact Strength Improvements
The improvement in impact strength of β-nucleated PP injection molded articles over their α crystalline counterparts is significant – often more than two-fold. This is useful, for example, in molded threaded closures such as bottles and caps.
Shi et al., US Patent 5,231,126
Cermak et al., “Relationship Between Mould Temperature and Properties of Injection-Moulded Pure and Beta Nucleated Polypropylenes”, SPE ANTEC 2004 proceedings.
Jacoby and Lee, “The Use of Beta Nucleation to Produce Microvoided Oriented Polypropylene Films”, SPE ANTEC 2005 proceedings.
Jacoby, “The Use of Beta Nucleation to Improve the Impact Strength and Reduce the Warpage of Polypropylene Caps & Closures”, SPE ANTEC 2009 proceedings.
Jacoby, “A New Beta Nucleant Masterbatch for Mineral Filled and Unfilled Polypropylene Applications”, SPE ANTEC 2011 proceedings.
Jacoby, “Recent Insights on the Use of Beta Nucleation to Improve the Thermoforming Characteristics of Polypropylene”, SPE ANTEC 2012 proceedings.
About the Author:
Dr. Moris Amon is a technical consultant in the fields of polymer processing, polymeric materials and intellectual property. He has advised clients in the packaging, labels, lighting, and construction industries since 2010. Dr. Amon holds a doctorate in chemical engineering from the University of Delaware. He has held research and development positions at the American National Can Co., ExxonMobil Films, and Avery Dennison Corp. In 2006, he was named a Fellow of the Society of Plastics Engineers. Dr. Amon is the inventor of 14 US Patents and is a USPTO-registered patent agent.
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